I don't think the size of the tank really matters. The rating on the tank is the only inportant part. With a 3000 PSI (48ci i'm guessing?), you should have enough air pressure to maintain any marker. Remember, it's not about how much air you hold in the tank, it's about the pressure that that air has on it. As long as your tank is still pressurized, it should be fine. You might try filling the tank more often (topping it off is usually not good, but you could try it for the sake of experimentation), or take the marker to your local shop and ask them to take a look at it. Sorry if neither of these is helpful. Hope it all works out! Sounds like you have a wicked setup.